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Research Papers

Rotating Instability in an Annular Cascade: Detailed Analysis of the Instationary Flow Phenomena

[+] Author and Article Information
Benjamin Pardowitz

German Aerospace Center (DLR),
Institute of Propulsion Technology,
Engine Acoustics Department,
Müller-Breslau-Straße 8,
Berlin 10623, Germany
e-mail: Benjamin.Pardowitz@dlr.de

Ulf Tapken, Lars Enghardt

German Aerospace Center (DLR),
Institute of Propulsion Technology,
Engine Acoustics Department,
Müller-Breslau-Straße 8,
Berlin 10623, Germany

Robert Sorge, Paul Uwe Thamsen

Technical University Berlin,
Fluiddynamics Department,
Straße des 17. Juni 135,
Berlin 10623, Germany

Institute for Aeronautics and Astronautics, Chair for Aero Engines.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 6, 2013; final manuscript received September 3, 2013; published online November 28, 2013. Editor: Ronald Bunker.

J. Turbomach 136(6), 061017 (Nov 28, 2013) (10 pages) Paper No: TURBO-13-1183; doi: 10.1115/1.4025734 History: Received August 06, 2013; Revised September 03, 2013

Rotating instability (RI) occurs at off-design conditions in compressors, predominantly in configurations with large tip or hub clearance ratios of s* 3%. RI is the source of the blade tip vortex noise and a potential indicator for critical operating conditions like rotating stall and surge. The objective of this paper is to give more physical insight into the RI phenomenon using the analysis results of combined near-field measurements with high-speed particle image velocimetry (PIV) and unsteady pressure sensors. The investigation was pursued on an annular cascade with hub clearance. Both the unsteady flow field next to the leading edge as well as the associated rotating pressure waves were captured. A special analysis method illustrates the characteristic pressure wave amplitude distribution, denoted as “modal events” of the RI. Moreover, the slightly adapted method reveals the unsteady flow structures corresponding to the RI. Correlations between the flow profile, the dominant vortex structures, and the rotating pressure waves were found. Results provide evidence to a new hypothesis, implying that shear layer instabilities constitute the basic mechanism of the RI.

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Figures

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Fig. 1

Integrated measurement equipment in the annular compressor cascade (unsteady pressure sensors and high-speed PIV)

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Fig. 2

Meridional view of the test section with radial dimensions (units: mm)

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Fig. 3

Positioning of the high-speed PIV system at the annular cascade

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Fig. 4

Averaged 2D flow fields in the annular cascade measured tangentially at the hub (radius ri=86 mm) for two incidence angles i1=+8 deg (top) and i2=+11 deg (bottom). Labeled positions (◻), see Fig. 5.

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Fig. 5

Radial profiles of the axial and azimuthal velocities for both flow incidence angles of i=+8 deg (left) and i=+11 deg (right) determined at the position x=514 mm and y=0 mm in Fig. 4

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Fig. 6

Spectral analysis of the unsteady flow field. Power spectral densities of velocity fluctuations measured with high-speed PIV (top). Circumferential mode amplitudes deduced from the pressure measurements (bottom).

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Fig. 7

Amplitude distribution of the dominant RI mode of order m = 5 based on the time-resolved mode analysis using the pressure measurements

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Fig. 8

Identified triggers N depending on the amplitude threshold Thrm and phase range ΦΔ for the analysis of 10 s of pressure data

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Fig. 9

Visualization of a modal event of the RI mode m = 5 using the analysis procedure (step 1–3) applied to the pressure data

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Fig. 10

Exemplary unsteady flow field at a radial position of r = 86 mm (next to the hub) for operating conditions where the RI is present

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Fig. 11

Filtered velocity fluctuations at two monitor regions I (solid lines) and II (dashed lines) with its magnitude (top), axial (middle), and tangential velocity components (bottom)

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Fig. 12

Number of identified triggers N depending on the amplitude threshold A'Thr and a phase range αΔ

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Fig. 13

Period of the unsteady flow field in correspondence to the RI mode m = 5 (see Fig. 14) at radius r = 86 mm (Nomenclature as in Fig. 10)

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Fig. 14

Unsteady pressure field of the modal event triggered by the dominant RI mode m = 5 corresponding to Fig. 13 (a single passage is highlighted; Nomenclature as in Fig. 9)

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